One of the two atrio-ventricular valves in the heart is the mitral valve, which is located on the left side of the heart and which forms or defines a valve annulus and valve leaflets. The mitral valve is located between the left atrium and the left ventricle, and serves to direct oxygenated blood from the lungs through the left side of the heart and into the aorta for distribution to the body. As with other valves of the heart, the mitral valve is a passive structure in that it does not itself expend any energy and does not perform any active contractile function.
The mitral valve includes two moveable leaflets that open and close in response to differential pressures on either side of the valve. Ideally, the leaflets move apart from each other when the valve is in an open position, and meet or “coapt” when the valve is in a closed position. However, problems can develop with valves, which can generally be classified as either stenosis, in which a valve does not open properly, or insufficiency (also called regurgitation), in which a valve does not close properly. Stenosis and insufficiency may occur concomitantly in the same valve. The effects of valvular dysfunction vary, with mitral regurgitation or backflow typically having relatively severe physiological consequences to the patient. Regurgitation, along with other abnormalities of the mitral valve, can increase the workload placed on the heart. The severity of this increased stress on the heart and the patient, and the ability of the heart to adapt to it, determine the treatment options that are available for a particular patient. In some cases, medication can be sufficient to treat the patient, which is the preferred option when it is viable; however, in many cases, defective valves have to be repaired or completely replaced in order to adequately restore the function of the heart.
One situation where repair of a mitral valve is often viable is when the defects present in the valve are associated with dilation of the valve annulus, which not only prevents competence of the valve but also results in distortion of the normal shape of the valve orifice. Remodeling of the annulus is central to these types of reconstructive procedures on the mitral valve. When a mitral valve is repaired, the result is generally a reduction in the size of the posterior segment of the mitral valve annulus. As a part of the mitral valve repair, the involved segment of the annulus is diminished (i.e., constricted) so that the leaflets may coapt correctly on closing, and/or the annulus is stabilized to prevent post-operative dilatation from occurring. Either result is frequently achieved by the implantation of a prosthetic ring or band in the supra annular position. The purpose of the ring or band is to restrict, remodel and/or support the annulus to correct and/or prevent valvular insufficiency. Such repairs of the valve, when technically possible, can produce relatively good long-term results.
However, valve repair is sometimes either impossible, undesirable, or has failed, such as in cases where the problem is not related to dilation of the valve annulus, leaving valve replacement as the most viable option for improving operation of the mitral valve. The two general categories of valves that are used for mitral valve replacement are mechanical valves and bioprosthetic or tissue valves. A wide variety of mechanical valves are available that accommodate the blood flow requirements of the particular location where they will be implanted; however, the use of these mechanical devices in the body can increase the risk of clotting in the blood stream, which can lead to a heart attack or stroke. Thus, mechanical valve recipients must take anti-coagulant drugs for the rest of their lives to minimize the potential of blood clots. The use of tissue valves advantageously eliminates the need for such anti-coagulant drugs; however, tissue valves do not typically last as long as mechanical valves and may need to be replaced at some later point in the patient's life. To implant either mechanical or tissue valves, a surgical procedure is typically used that involves opening the patient's chest to access the mitral valve through the left atrium, and then implanting the new valve in position.
To simplify surgical procedures and reduce patient trauma, there has been a recent increased interest in minimally invasive and percutaneous replacement of cardiac valves. Such a replacement of a heart valve typically does not involve actual physical removal of the diseased or injured native heart valve, but instead includes delivery of a replacement valve in a compressed condition to the native valve site, where it is expanded. One example of such a replacement procedure for a pulmonary valve includes inserting a replacement pulmonary valve into a balloon catheter and delivering it percutaneously via the vascular system to the location of a failed pulmonary valve. There, the replacement valve is expanded by a balloon to compress the native valve leaflets against the right ventricular outflow tract, thereby anchoring and sealing the replacement valve. In the context of percutaneous pulmonary valve replacement, U.S. Patent Application Publication Nos. 2003/0199971 A1 and 2003/0199963 A1, both filed by Tower, et al., describe a valved segment of bovine jugular vein, mounted within an expandable stent, for use as a replacement pulmonary valve. As described in the articles: “Percutaneous Insertion of the Pulmonary Valve”, Bonhoeffer, et al., Journal of the American College of Cardiology 2002; 39: 1664-1669 and “Transcatheter Replacement of a Bovine Valve in Pulmonary Position”, Bonhoeffer, et al., Circulation 2000; 102: 813-816, a replacement pulmonary valve may be implanted to replace native pulmonary valves or prosthetic pulmonary valves located in valved conduits. Other implantables and implant delivery devices also are disclosed in published U.S. Patent Application Publication No. 2003/0036791 A1 and European Patent Application No. 1 057 460-A1.
The percutaneous valve implantation procedures described above typically involve the movement of a compressed valve through at least some portion of the vasculature of the patient to the delivery site, and are therefore particularly well-suited for implanting relatively small valves, such pulmonary valves or aortic valves. Because a replacement mitral valve is typically relatively large as compared to the portions of the anatomy through which it would need to travel to reach the region of the native mitral valve, the percutaneous valve implantation procedures described in the above journal articles may not be feasible for a mitral valve. However, there is a continued desire to be able to be able to improve mitral valve replacement devices and procedures to accommodate the physical structure of the heart without causing undue stress to the patient during the operation on the heart, such as providing devices and methods for replacing the mitral valve percutaneously.